PROJECT SUMMARY Heart failure (HF) remains pre-eminent as a cause of mortality and morbidity in the U.S. Over the past half century several advances in the treatment of patients with HF (e.g., -adrenergic receptor blockade therapy) have improved survival, but only modestly. Since HF remains the most prominent cause of morbidity and mortality in this country, it follows that current therapeutic techniques are not adequate. This application is based on the concept that an increased understanding of adenylyl cyclase (AC) regulation in the heart will lead to novel therapeutic approaches. We are testing the hypothesis that the cardiac AC isoforms, AC5 and AC6, behave differently in the pathogenesis of HF leading to the concept that inhibition of AC5 may be a novel approach to the treatment of HF. Inhibition of AC5 may be superior to inhibition of -adrenergic receptor (-AR) stimulation, since it does not impair left ventricular (LV) function, yet may lead to more effective -AR desensitization. The apparent different actions of AC5 and AC6 are the cornerstones of this application. There are 3 major hypotheses: Hypothesis A: In the AC5 Tg mice, cardiac stress leads rapidly to LV decompensation, LV hypertrophy (LVH), and HF, whereas AC5 KO mice are protected. AC6 Tg are also thought to be protected, but the effects of chronic pressure overload and chronic catecholamine stress have never been examined in AC6 Tg mice. Our hypothesis is that AC6 Tg are relatively protected from chronic pressure overload and catecholamine stress compared with AC5 Tg. Hypothesis B: There are four mechanisms, which can reconcile the differences between AC5 Tg and AC6 Tg in response to stress (1) Although AC5 contributes roughly a third of cyclase activity normally, the AC5 Tg generates more AC activity than AC6 Tg for a given increase in transgene expression; (2) AC5 and AC6 generate differences in intracellular microdomains of cAMP, which are regulated by different subtypes of phosphodiesterase (PDE), e.g. PDE3 and PDE4; (3) differences in protein-protein interactions; and (4) differences in intracellular localizations. Hypothesis C: Oxidative stress is a major mechanism by which chronic pressure overload or catecholamine stress induces apoptosis and LV decompensation. Overexpression of AC5, but not AC6, increases oxidative stress and apoptosis, which can explain the differences in response to chronic pressure overload and chronic catecholamine stress in AC5 KO and AC6 Tg mice. The implications for Public Health are self-evident, considering that heart disease and HF are the disease processes which have the greatest impact on Public Health in the US, in terms of finances and task- force, and using similar logic, finding new therapies will be crucial to minimize the impact of these disease states on Public Health.